/Engine/Source/Runtime/Engine/Public/AnimationRuntime.h
Implementation should return the index into the PerBoneBlendData array that would be required when looking up/blending BoneIndex. This call will be passed the results of GetPerBoneInterpolationData, so the two functions should be matched.
int32 GetPerBoneInterpolationIndex ( const [FCompactPoseBoneIndex](API\Runtime\AnimationCore\FCompactPoseBoneIndex) & InCompactPoseBoneIndex, const [FBoneContainer](API\Runtime\Engine\FBoneContainer) & RequiredBones, const [FPerBoneInterpolationData](API\Runtime\Engine\IInterpolationIndexProvider\FPerBoneInterpolationData) * Data ) const